Two varieties of soybean (Glycine max L. cv. Clark and Crawford) showed a high sensitivity to low concentrations of diuron herbicide [3-(3,4-dichlorophenyl)-1,1-dimethyl urea]. Depending on the application dose of diuron, chlorosis, necrosis, and wilting of leaves ending in death after 7 to 10 days were observed. These symptoms were more obvious in the Clark variety even at the lowest concentration of diuron (1 ppm, 4.29 μM). Electron microscopic observation of treated plants revealed disorganization in the ultrastructure of mesophyll cell organelles. Disappearance of starch grains, decrease of plastoglobuli, and swelling and disarrangement of grana thylakoid of chloroplasts were observed in comparison to untreated plants. The guard cells of stomata of the Clark variety were seriously affected. The chlorophyll content of diuron-treated plants was sharply lowered. On the other hand, proline, soluble protein, and …